Alas, Poor Villain: The fact that Jennifer is possessed shows she's not in complete control of her actions and by doing the ritual on her, Low Shoulder is pretty much responsible for her death.
Alpha Bitch: She's a popular cheerleader who's also a massive bitch, to the point where the other students consider her "evil." Despite this, guys trip over themselves to impress her. She does have shades of a nicer side, at least at first, but only to Needy. However, Needy says she wasn't a good friend even when they were kids; she would steal her toys and pour lemonade on her bed.
Ambiguously Bi: Much like Needy, she shows many traits of this, actually trying to seduce her (and succeeding) at one point. (invokedMegan Fox says that she played Jennifer as a deeply closeted lesbian.) She also utters this great line:
Needy: I thought you only killed boys! Jennifer: I go both ways.
Berserk Button: Don't tell her about how she had to use laxatives to stay skinny back when she was alive.
Depraved Bisexual: Possibly. She "go[es] both way" by her own admission, but Megan Fox has stated that she viewed Jennifer as gay. But she still kills and eats people, and even before that, she was a Manipulative Bitch who bullied Needy.
Drunk on the Dark Side: Jennifer really enjoys her new powers after becoming possessed and seems to revel in the murders she commits. She wasn't evil before her change, but her turn to evil does involve the flaws she already had consuming her.
My Death Is Just the Beginning: In the final fight before her death, Jennifer bites Needy and in doing so transfers some of her demonic spirit and powers to Needy so that she may in a sense still live on through her.
Psycho Lesbian: Another possible interpretation of Jennifer's behavior. While she does seduce (and kill) boys, she is really only attached to Needy, and her flirtation and hooking up with Needy is contrasted with the lack of enjoyment she gets from boys.
Skinny Dipping: After eating Jonas, Jennifer goes for an after-meal swim in the lake while naked, presumably to wash off all the gore from her body.
Slasher Smile: Jennifer displays one at various times. Most notably after she first becomes possessed and when she appears in Needy's vision sitting on top of the chair watching her and Chip have sex.
So Beautiful, It's a Curse: Her stigma against boys was because she was treated as a sex object and (according to Megan Fox) she played up the sex appeal in order to conceal her feelings for Needy. Low Shoulder sacrificed Jennifer under the belief she was a virgin and her body was possessed a succubus, a being that saw her as the perfect vessel due to her beauty.
Straw Feminist: Jennifer has a bit of this trait, claiming that PMS was invented by "boy-run media to make (women) seem crazy". An unused exchange between her and Needy also has her trying to justify her murders thus:
Needy: You're killing people! Jennifer: No, I'm killing boys.
Tragic Villain: A textbook case where things beyond her control and her own fatal flaws prove her undoing.
Vitriolic Best Buds: She and Chip take snipes at each other, but Chip still ultimately considers her a friend and is immediately willing to let her be his rebound girl after Needy breaks up with him. Jennifer might have also felt something for him before becoming a monster.
Berserk Button: Don't tell her what to eat. An orderly lost a tooth when she recommended she eat more than Toastems.
Beware the Nice Ones: She's very sweet-natured and lovable but is not to be messed with as Jennifer finds out the hard way.
Break the Cutie: She starts as a kind but insecure Shrinking Violet, then has to deal with her best friend transforming into a monstrous demon, people around her dying, losing her boyfriend, then finally having to kill Jennifer and being locked up in a mental ward.
The Cassandra: She tries to tell everyone over and over that Jennifer is a monster, both before and after her death, and even after that, she tries to convince the psych ward staff about the devil sacrifice. Nobody believes her.
Cool Loser: She's supposed to be the "ugly best friend", but is clearly drop dead gorgeous and best friends with Jennifer, the most popular girl in school. It's later shown that she downplays her own looks and covers herself up so as not to outshine Jennifer.
Cursed With Awesome: When Needy kills Jennifer, she gains some of her powers, which boils down to: all the good ones, none of the bad. However, unlike Jennifer, Needy doesn't relish them but uses them to get her job done.
Face Death with Dignity: He impales Jennifer, which fails to kill her, but he still gets serious points for trying, and then, as he lies dying, he uses his last moments to try and comfort Needy, telling her that he loves her, and peacefully accepting that it's his time to go.
Nice Guy: He's kind and supportive of Needy, and seems to get along well with everyone except Jennifer, which isn't for lack of trying on his part and is almost totally on her.
Sacrificial Lion: His death is what makes Needy finally, really commit to killing Jennifer for once and all.
Vitriolic Best Buds: He and Jennifer take snipes at each other, but Chip still ultimately considers her a friend and is immediately willing to let her be his rebound girl after Needy breaks up with him. Jennifer might have also felt something for him before becoming a monster.
Jerkass: Even beyond his obvious evil deeds of killing an innocent girl for a record deal, he and his band go out of their way to mock Jennifer as she's dying, seemingly for no other reason than they feel like it.
Karma Houdini Warranty: He and his band get away with both starting the fire at the bar and their murder of Jennifer, becoming heroes in the public eye and boosting their success as a result. Then Needy comes calling, with the demonic powers their ritual had given Jennifer now inside of her, to kill them all as vengeance.
The Leader: Of his indie rock band, "Low Shoulder".